function ListNode(val, next) {
    this.val = (val === undefined ? 0 : val)
    this.next = (next === undefined ? null : next)
}

var mergeTwoLists = function (list1, list2) {
    let head = new ListNode()

    let dummy = head

    while (l1 !== null && l2 !== null) {
        if (l1.val < l2.val) {
            head.next = l1;
            l1 = l1.next;
        } else {
            head.next = l2;
            l2 = l2.next;
        }
        head = head.next;
    };

    if (l1 !== null) {
        head.next = l1;
    }
    if (l2 !== null) {
        head.next = l2;

    };
    return dummy.next;
}
//错哪了